The production of nearly all plastic films, tubing or pipe is performed by substantially the same process of heating the bulk plastic material to a soft stage, or melt, and then forcing it through a heated die assembly. The extrusion assembly includes a heated body containing a smooth tubular outer ring which forms the exterior surface of the extrudate and a rodlike coaxial inner core or mandrel that forms the smooth bore of the extrudate. In some instances the extrudate is first formed with a larger diameter and thicker walls than desired in the finished product and is then reduced and thinned by forcing the extrusion through a heated outer ring with a coaxial center mandrel. One problem encountered in the fabrication of such a die assembly is that of assuring that the mandrel remains precisely coaxial with the ring so that the wall thickness of the final extrudate is of a constant and equal thickness.
One type of die presently in use is referred to as a spider die which contains two or more small streamlined spacers that are fitted between inner mandrel and outer ring and in the flow path of the melt. The rejoined segments of the melt that passed between the spacers in the spider die, while invisible and apparently homogeneous in the final extrudate, form radial weld lines which tend to weaken the extrudate against radial forces.
Another type of die designed to overcome the poor melt distribution in the extrudates formed by the spider die is the spiral die which contains an inner mandrel and outer ring which spreads the fluid melt in spiral channels from a exterior of the truncated conical mandrel surface. A higher pressure is required to pump the melt through a spiral die because the very thick melt entering the die must be forced around right angled turns to enter and exit the spiral channels. The weld lines in an extrudate from a spiral die are spirals extending between the bore and outer surface of the extrudate, and because the melt from the channels overlaps there is greater distribution of the melt than of an identically appearing extrudate fabricated by the earlier described spider die.
The invention to be described herein is for an improved flow distributor between the inner mandrel and outer ring of a die assembly, the distributor producing extrudates with a single weld line that does not intercept either the bore or the exterior surface of an extrudate and has much greater bursting strength than either extrudates formed by spider dies or spiral dies. This makes the distributor not only superior for extruding pipes and tubing, but similarly superior for blown film applications.